Gas content analysis with a metal levitation system



April 21,1970 I w. M. HICKA M 3,507,144

' GAS commw ANALYSIS WITH A METAL 'LEVII'ATION SYSTEM Filed March 21,1968 l8 v 4 1e F|G.I.

FIG.2.

3s m as WITNESSES INVENTOR William M. Hickom United States Patent Int.Cl. G011! 7/14 US. CI. 7319 6 Claims ABSTRACT OF THE DISCLOSURE Systemfor precisely determining the gas content of a material using alevitation melting apparatus for extracting from a material sample thegas content of the sample including a closed melting chamber with aninert atmosphere in which only the sample under test is fused and nocomponent within the chamber is heated to any appreciable temperature,whereby no extraneous gases are introduced.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates to the art of extracting the gaseous content of a fusible sampleand for analysis of the gas content of the sample without contaminationor interference from extraneous gas sources. The apparatus comprises aspecially constructed levitation melting system.

Description of the prior art Materials such as metals may containvarious gases adsorbed or dissolved therein, retained inintercrystalline spaces, or distributed molecularly or atomicallytherein, or by a combination of several of such mechanisms. Thus siliconsteel may contain hydrogen, nitrogen, and oxygen as well as carbonmonoxide and carbon dioxide. It is desirable to determine the content ofthese gases in the steel since the magnetic properties will varymarkedly with some of these gases if in excess. For example, hydrogengas has caused failure of large steel shafts when present in excess ofcertain critical proportions. In nuclear fuel and reactor structuralelements, radioactive and byproduct gases need to be determinedaccurately. Consequently, there is a need to detect and analyze forthese gases in concentrations of less than one p.p.m.

A large number of analytical techniques are available for determiningthe concentration of gases in metals. These include hot extraction,Kjeldahl, combustion, vacuum fusion, spark source mass spectrometry,activation, isotopic dilution, and visual inspection. All of thesemethods have advantages and limitations dependent on such factors assampled composition, sample shape and size, time required for analysis,detection limit, concentration range required, cost per analysis, costof equipment, precision of results, etc. -In the use of most of thesetechniques it is extremely diflicult to obtain analysis of gases atconcentrations of one part per million and less.

Levitation melting is a technique for melting a small amount of metal orother electrically conductive substance while it is magneticallysuspended in a vacuum or in an inert gaseous atmosphere. The metalsample being melted is suspended within an electromagnetic fieldprovided by an induction coil or the like with current of highfrequency, such as about 450,000 hertz. The metal is heated by eddycurrent induced in it and is supported by repulsion between the inducededdy currents and the ind'ucing coil current. The metal sample rapidlyheats up and fuses as a result of these induced currents, with the metalbeing stirred by the electrical forces. An outstanding' advantage oflevitation melting is the removal of risk 3,507,144 Patented Apr. 21,1970 ice of contamination of the sample by .the exclusion of acontacting crucible or an electrode.

Levitation of a solid by means of an electromagnetic field has beenknown for some time. US. Patent Nos. 2,686,864 and 2,957,064 disclosedevices for the levitationrnelting of metal samples, and are typical ofthe prior known devices for levitation melting. Such devices normallyhave enclosed a large part of the apparatus including theradio-frequency levitation coil Within a bell jar or vacuum container inwhich the metal sample is levitated.

Many methods have been used to degas metals to some degree includinglevitation melting devices such as those disclosed in theabove-mentioned patents. In those systems the field producing coils arecontained within the vacuum, or inert atmosphere, in the bell jar orother confining means, the gases from the metal sample being melted areadmixed with the gases released from the coils and other metal partsassociated therewith. The heat radiated from the molten metal, and partsheated by induced currents causes gases to be released from any partsubject to their influence. For example, in a larger system the initialpressure requirement in the bell jar for analyzing the gas content of ametal sample may be of the order of 10- torr while the gas released fromthe sample may raise the pressure to 10 torr; that is, 10% of the, totalgas from the system is background or contaminant gas which is not partof the sample.

Associated with the foregoing has been the problem of setting up thesystem for the analysis of only a single sample. Most of the prior knownsystems for the levitation melting of metals have involved atime-consuming procedure of placing the metal sample Within'the system,sealing the system, and then providing the proper atmosphere (vacuum orinert gas) within the system before the sample is melted.

In accordance with this invention, it has been found that the foregoingdisadvantages may be overcome by providing a system in which thelevitation coils are external of the melting zone in which the metalsample is disposed. Also, in the melting zone of the system a holder fora plurality of samples may be provided to enable the testing of anydesired number of samples in a sequential manner, and thereby avoid thedelay involved in setting up the system for all of the samplesseparately.

The advantage of the system of the present invention is that the out-gaseffect of the system is reduced essentially to the sample and eliminatesthe out-gassing eifect of other metal parts such as the inductionheating coil which was formerly included in prior known levitationsystems. Moreover, the smaller the system, the smaller the surface areaof the surrounding that contributes to the out-gassing due to radiationheating from the heated sample.

Furthermore, in a smaller system a smaller sample may be used with theevacuated initial system pressure being of the order of 10' torr whichis raised to a pressure of about 10- torr when the sample is levitationmelted, with the background etfect being only 0.1%. Thus, the samplepressure is raised by a factor of 1000 above the background gas content.

Accordingly, it is a general object of this invention to provide a metallevitation gas extraction system so proportioned and arranged, with anexternal levitating coil, so as to provide for rapid and accurateheating of a sample to the melting temperature whereby to extract allthe gas and volatile content thereof, and to analyze the gas soextracted, without any substantial contamination heating and melting ofa plurality of samples consecutively SUMMARY OF THE INVENTION Briefly,the present invention consists of an elongated sealed container whichcommunicates with a means for evacuating and analyzing the gases evolvedin the container, and a levitation coil external of and surrounding thetubular container for creating an electromagnetic field for levitating,heating and melting a sample to be analyzed for its gas content withinthe container. The container is of such size and shape that nosubstantial amount of gas is evolved from any component other than thesample. The apparatus components are so arranged that no portion of theapparatus is subjected to sufiicient heat to evolve any appreciableamount of gas within the container enclosure except the sample beinganalyzed. In addition, the device may include a holder for mounting aplurality of samples at vertically spaced intervals within the tubularcontainer, and means for moving the tubular container or the heatingcoil longitudinally of the axis of the container to successivelylevitate and melt each sample within the container.

BRIEF DESCRIPTION OF THE DRAWING For a better understanding of thenature and objects of this invention, reference is made to the drawings,in which similar numerals refer to similar parts in the several views ofthe drawings, and in which:

FIGURE 1 is an elevational view of the levitation gas extraction systemof the present invention; and

FIG. 2 is an enlarged fragmentary view of the system showing the mannerin which the metal sample is levitated within the tubular container.

DESCRIPTION OF THE PREFERRED EMBODIMENT The device of the presentinvention is generally indicated at in FIG. 1. It includes a tube 12, aconduit having rigid end portions 14 and 16 and an intermediate flexibleportion 18, a joint 20 between the conduit portion 14 and the upper endof the tube 12, and a radiofrequency levitation coil 22.

The tube 12 is preferably circular in cross section and is an elongatedmember having a closed lower end 24. The upper end of the tube is openand communicates with the lower end of the conduit portion 14 to whichconnection is made by the joint 20 which is either a mechanical joint(O-ring or gold gasket) or involves a tapered fitted part as well asApiezon Wax for sealing the joint such as for a high vacuum condition.

The tube 12 is composed of a suitable high temperature material such asquartz, alumina, zirconia, or other high temperature oxides that can befabricated into useful configuration. For lower temperature operationsPyrex glass may be used. The preferred material is quartz, because ofits transparency as well as good high temperature and high vacuumproperties. The conduit is heated by an electromagnetic field-inducingmeans such as by a field induced within the conduit by a radio-frequencycoil 22.

As shown in FIG. 1, the end portions 14 and 16 of the conduit areprovided with valves 26 and 28. The right end of the tube portion 16 (asviewed in FIG. 1) is connected to a mass spectrometer which is not shownin the drawing.

The intermediate portion 18 of the conduit is preferably composed ofstainless steel. The intermediate portion is flexible to permit verticalmovement of the tube 12 within the coil 22 as will be describedhereinbelow.

As shown more particularly in FIG. 2, the coil 22 is a tubular memberthrough which coolant such as water is passed during operation. The coil22 preferably includes a lower portion 30 and an upper portion 32 whichportions are interconnected by an intermediate portion 34. The coil 22is preferably composed of a metal such as copper. The coil is powered bysuitable means such as a 5 lcw., 450 kc. generator.

Within the tube 12 means for supporting one or more samples to belevitated are provided and include an elongated rod 36 the lower end ofwhich rests upon the lower end 24 of the tube 12. At spaced locationsalong the rod 36 a plurality of sample-support holders or cups 38 aremounted on the rod so that samples to be levitated may be placed on eachholder before the rod 36 is placed in the tube 12. When in place, therod and holders 38 support the samples to be levitated in a positionsubstantially in line with the longitudinal axis of the tube 12. The rod36 and the holders 38 are preferably composed of a dielectric materialsuch as quartz.

OPERATION After being weighed and cleaned, the several samples 40, 42,and 44 are each placed on the spaced holders 38 as shown in thedrawings. The rod 36 is then placed within the tube 12 and the upper endis closed in a vacuum tight manner by the adaptor joint 20. The valves26 and 28 are then opened and the system is evacuated to a pressure ofthe order of from about 10- to 10* torr.

A coolant (such as water) is passed through the coil 22 and an RF powergenerator for the coil 22 is actuated. As shown in FIG. 2, theelectromagnetic field from the coil lifts the sample 42 from itsposition on the holder 38 into a position of support-free suspensionbetween the upper end lower coil portions 30 and 32 where the sample 42is heated and melted in the electromagnetic field generated by the coil.During its period of being molten, the sample 42 is continuously stirredby the RF field because of the circulating (eddy) currents in thesample. Due to the viscosity of the liquid metal as well as the RFfield, the molten metal assumes a spherical configuration. Thecontinuousstirring of the metal in the molten state causes a rapidrelease of the contained gas in the sample. The released gas isextracted by the use of any one of various types of pumps such as adiffusion pump, and turbo-molecular pump. The quantity of gas extractedis measured and analyzed by the use of a mass spectrometer or othermeans such as chemical, chromatographic, fractionation techniquesfollowed by pressure measurements in calibrated volumes to obtain theanalysis of the extracted gas.

After the gas is extracted from the sample the valve 26 is closed inorder to isolate the extracted gas from the tube 12 and, moreparticularly the molten sample, which when cooled, could reabsorb someof the extracted gas. The coil is then deenergized and the molten sampledrops into the holder 38 where it solidifies. The tube 12 is thentranslated by lowering through the coil in order to bring the nextsample 44 into the RF field for levitation, melting, and extraction ofthe gas.

The foregoing procedure is useful for actual gas analysis. However, thesystem may also be used for quality control of gas in metals. Isolationof the extracted gas may not be required and the mass spectrometer maybe replaced by a simple pressure measuring device such as thermocouple46 communicating with the tube portion 14 by a conduit 48. Thus ameasure of the gas content of the metal as required in manufacturing canbe determined by the included sirnple pressure measuring instrument 46such as a thermistor. The volume of the quartz tube 12 and theconnecting tube portion 14 are minimized to a volume of the order of,say, 50 to cc. by closing the valve 26. During levitation melting oftypical metals weighing 0.5 gram a pressure of the range of from torr to1 'torr of gas pressure would be developed in this small volume.

In addition, the system is suitable for isotopic dilution analysis forthe elements carbon, oxygen, nitrogen, and hydrogen. For that purpose,known quantities of high purity isotopes of C 0 N and D would be addedto the system prior to levitation of each sample. The ratio of C /C 0 /0N /N and H/D in the extracted gas provides the necessary information fordirectly calculating the quantities of each of these elements in theoriginal metal sample.

The following example illustrates the practice of the invention:

EXAMPLE The samples of approximately flj gram of copper were prepared byabrading the sample surfaces with a file and cleaning in benzenefollowed by acetone. The samples were placed in the system which wasthen evacuated to a pressure of the order of 10- torr. A preparedmixture of He and D was introduced at a pressure of approximately 1torr, and valve 26 was closed. The samples were levitated at roomtemperature and then heated to approximately 111200 C. for 2 minutes.The resulting gas mixtures were then extracted for each sample intoevacuated 3 liter volumes associated with the inlet system of the massspectrometer by opening valve 26.

Immediately on transfer of the gas and closing of valve 26, thegenerator was deenergized and the molten sample dropped to the holderwhere it solidified. It was found that once the quartz tube had beenconditioned by prebaking at 600 C. in vacuum, it could be reused aftershort term exposures to air or chemical cleaning without requiringfurther long term baking.

A typical mass spectrometer gas analyses obtained after levitationmelting copper samples in a D and He mixture are shown in the table. Thestarting sample mixture consisted of 20.75% D and 79.25% He. Sample No.2 was typical of the gas analyses obtained after levitation of highpurity vacuum arc melted copper in the gas mixture from which thehydrocarbon content of the sample is calculated. An advantage of thistechnique is that the sample is available for reanalysis.

TABLE .GAS ANALYSIS Sample H2 HD D2 He CO CO No. 1 Mixture 20. 75 79.N0. 2 0.5 gm. Cu 0. 61 1. 29 19. 10 77. 67 0. 91 0. 42

levitation coils and associated metal parts and to include only thesample or samples to :be analyzed. Moreover, the device of the presentinvention provides for the analysis of one or more samples for a givensetup for extracting and analyzing the gas from metal samples.

It is understood that the above specification and drawings are exemplaryof technically and economically feasible apparatus and methods forextracting and analyzing gases from metal samples.

What is claimed is:

1. Apparatus for analyzing gas extracted from an electrically conductivematerial by levitation melting in a controlled atmosphere, comprisingwalls forming a sealed melting chamber for at least one sample of thematerial, the walls being composed of a dielectric material, meanswholly external of the walls for establishing in the chamber anelectromagnetic field for levitating and melting the material sample,and means communicating with the melting chamber for withdrawing andanalyzing the gas evolved from the sample, whereby no extraneous membersare present in the electromagnetic field and within the melting chamberto evolve gas and contaminate the gas evolved from the sample underanalysis.

2. Theapparatus of claim 1 wherein the melting chamber is devoid ofelectrically conductive material.

3. The apparatus of claim 1 wherein the Walls forming the meltingchamber comprises a tubular member having a vertical longitudinal axis.

4. The apparatus of claim 3 wherein the means for establishing anelectromagnetic field includes a pair of vertically spaced inductioncoil means for inducing the field in a vertical zone substantiallycoaxial with the tubular member.

5. The apparatus of claim 3 wherein an elongated sample support memberis disposed within the tubular member and has a plurality of sampleholders disposed along the length thereof, which sample holders arespaced by a distance greater than the high intensity zone of the field.

6. The apparatus of claim 3 wherein one of the electromagnetic fieldmeans and the tubular member are vertically movable with respect to theother.

References Cited UNITED STATES PATENTS 2,664,496 12/1953 Brace 2l910.512,686,864 8/1954 Wroughton et al. 219- 2,686,865 8/1954 Kelly 219-12,957,064 10/1960 Comenetz 219-1049 3,055,206 9/1962 Watson et al. 73-19RICHARD C. QUEISSER, Primary Examiner a J. K. LUNSFORD, AssistantExaminer U.S. Cl. X.R. 219-7.5

